1. Field of the Invention
The present invention relates to method and apparatus for estimating muscle mass of a subject, and more particularly to, method and apparatus for readily estimating muscle mass with higher precision by mitigating any effect of extracellular water which may introduce some error in estimation of muscle mass.
2. Prior Art
There have been known many types of method of measuring muscle mass of a subject, an example of which is: Magnetic Resonance Imaging (hereafter referred to as “MRI”); Computer Tomography (“CT”); Dual Energy X-ray Absorptiometry (“DEXA”); and the like. Although the technique of MRI, CT and DEXA can provide precise measurement for muscle of the subject it requires very large scale apparatus and facilities, which makes impractical an application of such technique to small scale medical facilities or consumer devices for home use. Additionally, because of X-ray used in CT and DEXA there may be some possibility of exposure to X-ray.
Recently, a muscle estimation and calculation method has been proposed in which a weak alternating current is applied to a body of a subject and bioelectrical impedance is measured, based on which muscle mass is estimated and calculated. In particular, in this method, amount of water contained in the body (hereafter referred to as “total body water”) of the subject is calculated based on the measured bioelectrical impedance, and then, a fat free mass is calculated on the basis of the amount of total body water thus calculated. Thereafter, bone mass which is also calculated based on the bioelectrical impedance is subtracted from the fat free mass to produce a value which is considered as the muscle mass. This method can successfully be performed in more simple apparatus and facility than that for the case of MRI, CT and DEXA, as described above, so that it is suitable for use in small scale medical facility or even at home (see Patent Document 1, for example).
The patent document associated with the present invention is as follows:                Patent Document 1: International Laid-Open No. 01/015600 (page 23)        
The total body water generally consists of intracellular water that is present inside a cell membrane and extracellular water that is present outside the cell membrane. The extracellular water tends to greatly vary in amount depending on daily life of a person such as water intake, sweating, urination, etc., as compared to the intracellular water. Therefore, the method of estimating muscle mass based on the amount of total body water that is resulted from bioelectrical impedance, as described above, has such tendency that the result of estimation produced thereby depends on the amount of extracellular water at that moment (this is called “fluctuations within a day”), which makes difficult the precise estimation.
In view of the above it is an object of the present invention to provide method and apparatus for estimating muscle mass as simply as possible with higher precision by mitigating any effect of extracellular water which may introduce some error in estimation of muscle mass.